While in summertime, rainstorms do occur occasionally. Talking about rainstorms, one may recall the widespread heavy rain in Hong Kong on 8 May 1992. It caused more than 200 cases of flooding and numerous landslips, paralysing road traffic. Some roads in Admiralty were turned into raging rapids and many places were heavily flooded. Some people were washed by the rapids to low-lying areas and a few people were even found dead. This rainstorm should be well within the memory of the seniors. It was indeed after this rainstorm that the Observatory introduced the rainstorm warning system.
On 22 July this year, under the influence of an active southwesterly airstream, the weather in Hong Kong was unsettled with heavy rain and squally thunderstorms. The western part of Hong Kong Island was affected most, with over 300 millimetres of rainfall recorded throughout the day (Figure 1). The Amber Rainstorm Warning Signal issued by the Observatory lasted for nearly 12 hours and the Landslip Warning was also in force for more than 6 hours.
Figure 1 Rainfall distribution in Hong Kong on 22 July.
What was special about the rainband affecting Hong Kong in this episode was that the area of intense rain was relatively small but stayed at the same location for quite a long time. This was like an adhesive bandage sticking to the southern part of Hong Kong, leading to localized heavy rain over there but not much rainfall in the New Territories. The radar image in Figure 2 shows that the area of intense rain remained almost stationary over the southern part of Hong Kong for a prolonged period in the morning, whereas rain was less intense in the New Territories. Let us take 7:45 to 8:45 a.m., which was one of the periods with heavier rain, as an example. In this hour, 6 to 89 millimetres of rainfall were recorded by various raingauges in Southern District, while 28 to 72 millimetres were recorded by the raingauges in Central and Western District. In Wanchai, the rainfall ranged from 42 to 63 millimetres. Other districts to follow were Islands District, Eastern District, Yau Tsim Mong and Kowloon City, where the hourly rainfall was less than 50 millimetres. Rain was even much less at some places in Kowloon and most parts of the New Territories, over which less than 10 millimetres of rainfall were recorded in one hour. It can be seen from these figures that rain varied greatly from one place to another and torrential rain was confined to some places in the southern part of Hong Kong.
Figure 2 Weather radar images on 22 July at (a) 5:36 a.m. and (b) 9:00 a.m.
Heavy rain is indicated by orange and yellow colours.
The issuance of rainstorm warning signal by the Observatory is mainly based on the amount of rain which has fallen or is expected to fall generally over Hong Kong in an hour. The hourly rainfall criteria for the Amber, Red and Black signals are 30, 50 and 70 millimetres respectively. On 22 July morning, the overall rain intensity in Hong Kong met the criterion of the Amber Rainstorm Warning. Although the hourly rainfall over certain areas of the southern part of Hong Kong was more than 50 millimetres, the Observatory did not further upgrade the rainstorm warning level because heavy rain of such intensity did not extend to greater parts of Hong Kong. Nevertheless, in view of the risk of landslips arising from heavy rain, the Landslip Warning was issued at 10:45 a.m. The Observatory also maintained close liaison with relevant government departments and provided them with rainfall information to facilitate their emergency response and rescue work.
Heavy rain can vary abruptly and randomly. To give an accurate forecast of heavy rain is indeed a great challenge. The rainstorm warning system is designed to alert the public about the potential impact of heavy rain, and to ensure a state of readiness within the essential services to deal with emergencies. Besides knowing the level of rainstorm warning in force, members of the public should also be aware of the situation where they are, be vigilant and take suitable precautions to ensure their safety.
Hong Kong is situated on the south China coast, with mainland China to its north and the South China Sea to the south. When tropical cyclones come from the sea and make landfall over the south China coast, they will usually continue to move inland, weaken progressively and dissipate. Some tropical cyclones approach Hong Kong from the east and travel close to the coast of southern China. If the direction of their westward movement changes even slightly (Figure 1), not only will there be a difference in how they make landfall, but there can also be substantial difference in their intensity variations. How can such a small change lead to a large difference?
For a tropical cyclone over the sea to the east of Hong Kong, if it adopts a westerly track with a slight northerly component, it will hit the eastern part of the south China coast sooner (Track 1 in Figure 1; Figure 2), and travel inland for some distance before moving to the north of Hong Kong. That means, the tropical cyclone will weaken over the land for some time before approaching Hong Kong. By that time, it will have weakened considerably and its threat to Hong Kong will be relatively low. Conversely, if its westward track bears a slight southerly component, the tropical cyclone can stay very close to the coast without making landfall, or re-enter the sea after hitting the land (Track 2 in Figure 1). Either way, it can maintain its intensity while edging closer to Hong Kong, posing a greater threat to Hong Kong as demonstrated by Typhoon Maggie that hit the territory in 1999 (Figure 3). Back then, the Observatory issued the Tropical Cyclone Warning Signal No. 9 and Maggie made landfall over the Sai Kung peninsula, bringing gale force winds and heavy rain to Hong Kong. Due to sheltering by terrain, the northerly winds brought by tropical cyclone from the east will usually be relatively weak at first, but strengthen abruptly as the tropical cyclone comes very close to Hong Kong. The case of Maggie clearly illustrated that northerlies in Hong Kong strengthened from strong winds to gales within less than 3 hours (Figure 4). It can readily be seen that for tropical cyclones from the east, even if there is a tiny change in their direction of westward movement, their impact on the weather of Hong Kong can be drastically different.
Figure 1 A small change in the direction of westward movement of tropical cyclone can lead to a change in
how it makes landfall and a substantial difference in its intensity variations.
Figure 2 In September 2001, Tropical Cyclone Nari made landfall at Shantou and then weakened gradually.
It became a tropical depression before it was closest to Hong Kong.
Figure 3 Maggie moved westwards along the south China coast and maintained the intensity as a typhoon
prior to its landfall at Hong Kong.
Figure 4 Local winds strengthened from strong winds to gales within less than 3 hours
when Maggie came very close to Hong Kong.
Linfa that affected Hong Kong in early July 2015 is also an example of tropical cyclones coming from the east. In the morning of 9 July, Linfa was centred over the sea at more than 200 km to the east of Hong Kong. Being a typhoon with intact structure, it headed towards Hong Kong on a westward track. The fixed-wing aircraft jointly sent by the Observatory and the Government Flying Service recorded hurricane force winds in the vicinity of Linfa's eye, and gales of Linfa also extended out to about 100 km from its centre. As Linfa was forecast to traverse along the coast instead of moving inland, its intensity was anticipated to sustain. Local winds might also pick up quickly within a short period of time when Linfa moved further close to Hong Kong. It turned out that while Linfa skirted at just about 50 km to the north of the Observatory (Figure 5), it rapidly weakened and its gales did not affect Hong Kong generally. This formed a stark contrast to what happened with Tropical Cyclone Utor in 2001 (Figure 6). Although Utor weakened into a severe tropical storm following its landfall near Shanwei and skirted to the north of the Observatory at around 80 km inland, it was still able to bring gale force winds and heavy rain to Hong Kong for a prolonged period.
Figure 5 Linfa rapidly weakened before arrival and skirted at just about 50 km to the north of the Observatory.
Figure 6 Utor moved over the inland areas at about 80 km to the north of the Observatory and brought
gale force winds and heavy rain to Hong Kong.
Although the weather conditions and scientific factors in the above cases are different, all of them demonstrate that a small change can lead to a large difference. Therefore, forecasting typhoons from the east is full of challenges and their threat to Hong Kong should not be taken lightly. These cases deserve more research studies and they also serve as prime examples for considering the issuance of tropical cyclone warnings in the future.
Entering July this year, tropical cyclone activities over the western North Pacific have been increasing. The satellite imagery in Figure 1 shows that there are a total of three tropical cyclones over the northeastern part of the South China Sea and the western North Pacific. You may wonder: If Chan-hom continues to edge closer to Linfa, what will be the impacts on the latter? Concurrent occurrences of three tropical cyclones happened in the past, but were not many. Some examples in recent years can be found in the HKO's educational material.
Figure 1 Visible imagery captured at 8 a.m. on 6 July 2015 showing tropical cyclone Linfa over the northeastern part
of the South China Sea as well as Chan-hom and Nangka over the western North Pacific.
(The image was originally captured by MTSAT-2 of the Japan Meteorological Agency)
Dating back to the 20's to 30's of the last century, Dr. Sakuhei Fujiwhara (1884 - 1950) already discovered that when two tropical cyclones approach each other, they tend to rotate anti-clockwise about a point between them. This phenomenon is usually known as the Fujiwhara effect. Research studies showed that tropical cyclones begin to interact more prominently when their centers come within around 1200 km of each other. The degree of interaction increases as the separation distance decreases, while the separation distance where interaction commences depends on the sizes of the tropical cyclones. Other research studies also pointed out that the interaction between two tropical cyclones depends on the sizes and intensities of the tropical cyclones as well as the environmental steering flow; whereas tropical cyclones of unequal sizes likely to have greater interaction than two of similar sizes.
When two tropical cyclones come into proximity, it may bring about the following situations:
(1) The two tropical cyclones rotate in a stable orbit (Fujiwhara effect), followed by a release and escape (Figure 2). For example in 2009, tropical cyclone Parma near the Philippines interacted with another tropical cyclone Melor, and Parma underwent a looping motion during 5-7 October (Figure 3 and 4).
Figure 2 A conceptual model showing interaction of two tropical cyclones: approach and capture, followed by a stable mutual orbit (Fujiwhara effect), then release and escape. The diagram is adapted from Lander and Holland (1993).
Figure 3 The tracks of tropical cyclones Parma and Melor in September-October 2009.
Figure 4 Animation of infra-red satellite imagery showing tropical cyclones Parma and Melor from 3 to 12 October 2009. (HKT is local time, the satellite imagery was originally captured by MTSAT-2 of the Japan Meteorological Agency)
(2) One tropical cyclone is captured and "swallowed up" by another tropical cyclone, or the two tropical cyclones undergo a merger (Figure 5). This scenario is most likely to occur when one tropical cyclone is much larger and stronger than the other. Examples are Zeb and Alex in 1998 (Figure 6); Namtheun in 2010 near the Taiwan Strait interacting with Lionrock over the northeastern part of the South China Sea, weakening and dissipating afterwards (Figure 7 and 8).
Figure 5 A conceptual model showing interaction of two tropical cyclones. When they orbit and come closer and closer together,
they eventually undergo a merger. The diagram is adapted from Lander and Holland (1993).
Figure 6 Animation of infra-red satellite imagery showing tropical cyclone Zeb and Alex from 10 to 13 October 1998. (UTC is Coordinated Universal Time, the satellite imagery was originally captured by GMS of the Japan Meteorological Agency.
The animation is adapted from the University Corporation for Atmospheric Research (UCAR))
Figure 7 The tracks of tropical cyclones Lionrock, Namtheun and Kompasu in August - September 2010.
Figure 8 Animation of infra-red satellite imagery showing tropical cyclones Lionrock, Nantheun and Kompasu
from 30 August to 1 September 2010. (HKT is local time, the satellite imagery was originally captured by
MTSAT-2 of the Japan Meteorological Agency)
(3) The two tropical cyclones only exhibit "semi-direct" interaction, and their tracks are largely determined by the steering flow associated with other synoptic weather systems (Figure 9).
Figure 9 A conceptual model showing other synoptic weather system (e.g. subtropical ridge) provides the main steering flow to the tropical cyclones. The diagram is adapted from Carr and Elsberry (1997).
Whenever there are interactions between two or more tropical cyclones, they will drag each other, rotate, cause one of them weakening, merge together or escape from each other, etc. These are superimposed on the steering flow of the synoptic environment. The tracks of the tropical cyclones will then become rather complex, making forecasts more difficult. Nowadays, we have a grasp of the basic conceptual models, and computer numerical models can also generally capture the interaction processes between tropical cyclones. However, as many factors come into play (including changes in intensities, sizes and relative positions, etc. of the tropical cyclones), there will be discrepancies between different model forecasts, posing great challenges to forecasters. In any case, members of the public are advised to pay close attention to the latest tropical cyclone information and weather forecasts issued by the Observatory.
T Kung and C C Lam
 Brand, S., 1970: Interaction of binary tropical cyclones of the western North Pacific. J. Appl. Meteor.,9, 433-441.
 Introduction to Tropical Meteorology, 2nd Edition, Chapter 8: Tropical Cyclones, MetEd (2010) (http://meted.ucar.edu/), COMET Program, UCAR.
 Prieto, R., B. D. McNoldy, S. R. Fulton, and W. H. Schubert, 2003: A classification of binary tropical cyclone-like vortex interactions. Mon. Wea. Rev., 131, 2656-2666.
 Lander, M. A., and G. J. Holland, 1993: On the interaction of tropical-cyclone-scale vortices. I: Observation. Quart. J. Roy. Meteor. Soc., 119, 1347-1361.
 Carr, L.E., III, and R. L. Elsberry, 1997: Objective diagnosis of binary tropical cyclone interactions for the western North Pacific basin. Mon. Wea. Rev., 126, 1734-1740.